1. Field of the Invention
The present invention relates to a roller bearing and in particular to the art of reducing vibration and noise that are produced when the roller bearing is used.
2. Discussion of Related Art
There is widely known a roller bearing that includes a plurality of rollers provided between an outer circumferential surface of an inner member and an inner circumferential surface of an outer member; and a roller holder that has a plurality of pockets holding the plurality of rollers, respectively, such that each of the rollers is rotatable about an axis line thereof, and that is rotatable about the inner member. In this roller bearing, it is essentially needed to provide a gap around each of the rollers, held by a corresponding one of the pockets, so as to allow the each roller to rotate about its axis line. In addition, it is essentially needed to provide a gap between an inner circumferential surface of the roller holder and the outer circumferential surface of the inner member, and a gap between an outer circumferential surface of the roller holder and the inner circumferential surface of the outer member, so as to allow the roller holder to rotate about the inner member. Owing to the presence of those gaps, each of the rollers cannot be prevented from being inclined in a circumferential direction of the roller holder, and an angle of inclination of the axis line of the each roller relative to an axis line of the inner member in the circumferential direction is called “a skew angle”. Because of the presence of the skew angle, the each roller tends to displace, while rolling, in an axial direction of the inner member, as the inner and outer members rotate relative to each other. Thus, an axial-direction force is exerted to the roller holder, and respective axial-directions forces are exerted to the inner and outer members in respective opposite directions. The axial-direction force exerted to the roller holder changes its direction depending upon whether a sign of the skew angle is positive or negative. Since the sign (positive or negative) of the skew angle changes randomly, the direction of the axial-direction force changes randomly. This leads to a problem that an apparatus, such as a pump, including the roller bearing produces vibration and/or noise.
Patent Document 1 (Japanese Patent Application Publication No. 11-344,035) discloses a roller bearing including a roller holder, wherein an inclination of each pocket in a circumferential direction of the roller holder is not greater than 1.5/1000 of a length of each roller. In this roller bearing, a skew angle of each roller is reduced and accordingly vibration and noise produced by the bearing are reduced.
In addition, Patent Document 2 (Japanese Patent Application Publication No. 10-318,265) discloses a method of forming a roller holder by injection molding of a synthetic resin containing toughened fibers. In the disclosed method, a gate is provided at one of axially opposite ends of the roller holder, so as to increase a strength of the roller holder against concentration of stress to corners of pockets. Moreover, Patent Document 3 (Japanese Patent Application Publication No. 7-127,645) discloses a needle bearing employing a plurality of separated roller holders.